Many people already know that city trees are highly desirable to make a neighborhood more attractive. It is common knowledge that shade from trees keeps temperatures on the ground cooler, can help buildings lower their overall energy usage, and their roots absorb storm water runoff efficiently. What people may not be familiar with is that trees also aid in fighting crime. Yes, trees can lower crime rates.

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Researchers from the US Forest Service’s Pacific Northwest (PNW) and Southern Research Stations have published a study on this issue in the journal Environment and Behavior. Their theory is that certain types of trees can reduce crime in a given area. Their study focused on the neighborhoods of Portland, Oregon.

The total sample size was 2,831 single-family homes and the study period lasted between 2005 and 2007. During that time, 431 crimes were reported: 394 property crimes and 37 violent crimes. The researchers analyzed the relationship between trees and the various crime types.

They looked at many factors such as the number and size of trees per lot and trees in the surrounding area. They also factored in canopy size of street and yard trees. They concluded that large trees were associated with a reduction in crime. Meanwhile, numerous small trees were associated with an increase in crime.

"We believe that large street trees can reduce crime by signaling to a potential criminal that a neighborhood is better cared for and, therefore, a criminal is more likely to be caught," said Geoffrey Donovan, research forester with the PNW Research Station. "Large yard trees also were associated with lower crime rates, most likely because they are less view-obstructing than smaller trees."

Smaller trees can provide cover for criminals and block views from windows. Homeowners can rectify this by pruning the lower branches of trees and better placing the trees while planting. And let your trees grow tall!